Diffentiate "no backend" from "none resolved" in VDI_Resolve()

recover a minor improvement from varnishcache#2680:

This commit is to emit slightly different error messages to
differentiate between the cases "no backend set in vcl" and
"none returned from the director".

Includes minor polishing

Ref varnishcache#2860 varnishcache#3311

bin/varnishd/cache/cache_director.c

@@ -103,24 +103,26 @@ VRT_VDI_Resolve(VRT_CTX, VCL_BACKEND d)
 static VCL_BACKEND
 VDI_Resolve(VRT_CTX)
 {
-	const struct director *d;
+	VCL_BACKEND d;
 	struct busyobj *bo;
 
+	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
 	bo = ctx->bo;
 	CHECK_OBJ_NOTNULL(bo, BUSYOBJ_MAGIC);
-	CHECK_OBJ_ORNULL(bo->director_req, DIRECTOR_MAGIC);
-	d = VRT_VDI_Resolve(ctx, bo->director_req);
-	CHECK_OBJ_ORNULL(d, DIRECTOR_MAGIC);
-	if (d == NULL) {
-		VSLb(bo->vsl, SLT_FetchError, "No backend");
-	} else {
-		AN(d->vdir);
 
-		if (d->vdir->admin_health->health == 0)
-			d = NULL;
+	if (bo->director_req == NULL) {
+		VSLb(bo->vsl, SLT_FetchError, "No backend");
+		return (NULL);
 	}
 
-	return (d);
+	CHECK_OBJ(bo->director_req, DIRECTOR_MAGIC);
+	d = VRT_VDI_Resolve(ctx, bo->director_req);
+	if (d != NULL)
+		return (d);
+
+	VSLb(bo->vsl, SLT_FetchError,
+	    "Director %s returned no backend", bo->director_req->vcl_name);
+	return (NULL);
 }
 
 /* Get a set of response headers -------------------------------------*/